Methods and apparatus for an LED light engine

ABSTRACT

An LED light engine comprising a high thermal conductivity substrate (e.g., a metal-clad PCB), a plurality of light-emitting-diode (LED) semiconductor devices mechanically connected to the substrate, an outer dike fixed to the substrate and surrounding at least a portion of the LED devices, and a substantially transparent polymeric encapsulant (e.g., optical-grade silicone) disposed on the plurality of LED devices and restrained by said outer dike. In one embodiment, the light engine includes a reflector (e.g., a generally conic reflector) fixed to the substrate to form the outer dike. In another embodiment, an optical component (e.g., a lens, filter, or the like) is optically coupled to the polymeric encapsulant disposed on the LED devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent ApplicationNo. 60/507,858 filed Oct. 1, 2003, and 60/540,743, filed Jan. 30, 2004,both of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to lighting products and, moreparticularly, to light engines incorporating light-emitting diodes(LEDs).

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) have been used for decades in applicationsrequiring relatively low-energy indicator lamps, numerical readouts, andthe like. In recent years, however, the brightness and power ofindividual LEDs has increased substantially, resulting in theavailability of 1 watt and 5 watt devices.

While small, LEDs exhibit a high efficacy and life expectancy ascompared to traditional lighting products. For example, a typicalincandescent bulb has an efficacy of 10-12 lumens per watt, and lastsfor about 1000 to 2000 hours; a general fluorescent bulb has an efficacyof 40 to 80 lumens per watt, and lasts for 10000 to 20000 hours; atypical halogen bulb has an efficacy of 20 lumens and lasts for 2000 to3000 hours. In contrast, red-orange LED can emit 55 lumens per watt witha life-expectancy of about 100,000 hours.

Notwithstanding recent advances in LED efficiency, and the promise ofdramatic energy savings, known systems have failed to capitalize on theLED's desirable characteristics and produce systems that can replacestandard lighting products used in the commercial and consumer realms.This is primarily due to the limitations inherent in currently knownlight engines.

For example, commercial high power LED devices generate an enormousamount of heat—on the order of about 50 W/cm². In order to achievereliability and long life, it is important to keep the temperature ofthe LED devices fairly low. Currently known systems have failed toassemble multiple LEDs in a compact fashion while maintaining thenecessary heat transfer characteristics.

Similarly, it is desirable to protect the LED die with some form ofcoating, but it is difficult to reliably protect an array of multipleLED die using a standard semiconductor passivation as the thermalstresses resulting from temperature excursions (particularly in largescale assemblies) can caused sheared wire bonds, fractured die bonds,and other reliability problems.

Furthermore, efforts to incorporate multiple color LEDs to produce whitelight have been undesirable because, even when the LED devices areassembled in close proximity (which is again limited by heat transferconsiderations), the light produced by such systems is not well mixed,resulting in uneven blotches of individual colors rather than uniformprojection of white light. Similarly, current production compoundsemiconductor LED colors cannot produce certain wavelength efficiently(e.g., 575 nm yellow light). Mixing of efficient red and green LED lightis a better approach.

Accordingly, there is a need for LED light engine devices that overcomethese and other limitation of the prior art.

SUMMARY OF THE INVENTION

In general, the present invention provides a novel, multi-chip-on-board(MCOB) light engine comprising a high thermal conductivity substrate, aplurality of light-emitting-diode (LED) semiconductor devicesmechanically connected to the substrate, an outer dike fixed to thesubstrate and surrounding at least a portion of said LED devices, and asubstantially transparent polymeric encapsulant (e.g., optical-gradesilicone) disposed on the LED devices and restrained by the outer dike.

In accordance with one embodiment of the present invention, the highthermal conductivity substrate comprises a metal-clad printed circuitboard (PCB).

In accordance with various embodiments of the present invention, the LEDdevices are electrically configured in series, in parallel, or acombination thereof.

In accordance with an alternate embodiment of the present invention, thelight engine includes a reflector (e.g., a generally conic reflector)fixed to the substrate to form the outer dike.

In accordance with yet another embodiment of the present invention, anoptical component (e.g., a lens, filter, or the like), is opticallycoupled to the polymeric encapsulant disposed on the LED devices.

In this way, the present invention provides a high-efficiency LED lightengine suitable for a wide range of lighting applications.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description when considered in connection withthe Figures, where like reference numbers refer to similar elementsthroughout the Figures, and:

FIG. 1 is an isometric overview of a light engine in accordance with oneembodiment of the present invention having a plurality ofsurface-mounted LED chips configured in parallel and series;

FIG. 2 is a top view of a light engine in accordance with an alternateembodiment of the present invention having a plurality of wire-bondedLED chips configured in parallel and series, wherein the LED chips eachinclude two bond pads;

FIG. 3 is a top view of a light engine in accordance with an alternateembodiment of the present invention having a plurality of wire-bondedLED chips configured in series;

FIG. 4 is a top view of a light engine in accordance with an alternateembodiment of the present invention having a plurality of wire-bondedLED chips configured in parallel and series, wherein the LED chips eachinclude a single bond pad;

FIG. 5 is an isometric cut-away view of an exemplary light enginecomprising an LED die mounted on a metal-clad high-thermal-conductivityPCB substrate;

FIG. 6 is an isometric overview of a light engine including an innerdike and an outer dike;

FIGS. 7A and 7B show top and side views, respectively, of a light engineincluding an outer and inner dike filled with an encapsulant material;

FIG. 8 is an isometric overview of a light engine including a reflectorand an inner dike;

FIGS. 9A and 9B are top and side views, respectively, of the lightengine illustrated in FIG. 8;

FIGS. 10A and 10B are top and side views, respectively, of a lightengine incorporating an exemplary lens; and

FIG. 11 is a graph showing the spectra of various temperatures of whitelight.

DETAILED DESCRIPTION

The following description is of exemplary embodiments of the inventiononly, and is not intended to limit the scope, applicability orconfiguration of the invention in any way. Rather, the followingdescription is intended to provide a convenient illustration forimplementing various embodiments of the invention. As will becomeapparent, various changes may be made in the function and arrangement ofthe elements described in these embodiments without departing from thescope of the invention.

Overview

In general, an LED light engine in accordance with the present inventioncomprising a high thermal conductivity substrate (e.g., a metal-cladPCB), a plurality of light-emitting-diode (LED) semiconductor devicesmechanically connected to the substrate, an outer dike fixed to thesubstrate and surrounding at least a portion of (preferably all of) theLED devices, and a substantially transparent polymeric encapsulant(e.g., optical-grade silicone) disposed on the plurality of LED devicesand restrained by the outer dike. In one embodiment, the light engineincludes a reflector (e.g., a generally conic reflector) fixed to thesubstrate to form the outer dike and to assist in directing and focusinglight and/or mixing of light from two or more LED devices havingdifferent colors. In other embodiments, as discussed further below, oneor more optical components such as filters, lenses, and the like arefixed to the encapsulant coating.

LED Connectivity

Referring to FIG. 1, which shows an exemplary electrical topologyapplicable to the present invention, light engine 100 includes aplurality of LED devices 104 (in this embodiment, surface-mount LEDchips) connected to a high thermal conductivity substrate (or simply“substrate”) 102. In this embodiment, substrate 102 includes aconductive trace pattern 106 to which the plurality of LED devices 104are electrically and mechanically connected.

Trace pattern 106 is configured to interface with an AC or DC powersource, depending upon the application. For example, in the illustratedembodiment, a DC V₊ terminal 108 and a V_(o) terminal 110 are provided.These terminals are, in some instances, more generally referred toherein as the “input”.

LED devices 104 are electrically interconnected in any suitable manner.As shown in FIG. 1, for example, LED devices 104 may be configured in acircuit such that sets of individual devices are connected in series,wherein these sets are themselves connected in parallel with respect tothe input. In the illustrated embodiment, seven parallel columns, eachincluding five series-connected LED devices, are themselves connected inparallel with across terminals 108 and 110. Alternatively, withmomentary reference to FIG. 3, the plurality of LED devices 104 (in thisembodiment, 49 wire-bonded chips) are connected in series with respectto terminals 110 and 108.

In general, notwithstanding the illustrated embodiments described above,the present invention comprehends the use of any number of LED devicesconfigured in any suitable electrical topology (series, parallel, or acombination thereof) and any suitable geometry. For example, the LEDdevices may be positioned in a rectilinear pattern (a square orrectangular array, for example), a circular or curvilinear pattern, arandom or stochastic pattern, or any combination thereof. Furthermore,the LED devices may be laid out in multiple regions, where each of theregions exhibit different patterns and numbers of devices.

The number of LED devices 104 incorporated into the device may beselected in accordance with a number of design variables, including, forexample, the nature of the power source (AC converted to DC, availableDC voltage, available power, etc.), the nature of the LED devicesthemselves (e.g., forward voltage (V_(f)), power rating, etc.), thedesired color combination (described below), the nature of substrate 102(e.g., thermal conductivity, geometry, etc.), and the nature of theapplication and external thermal conditions.

In one embodiment, the LED devices are connected in series or parallelsuch that the overall combined forward voltage of the LED devicesmatches the electrical input. For example, in a household application inUS and Canada, 120 VAC must be rectified to 162V DC before can be inputto LED's. Normally, 40 to 80 LED devices can be connected in series,depending upon the V_(f) of the individual LEDs, to take the input of162V rectified DC. As is known, typical red and amber LED devices have anominal V_(f) of about 1.8 to 2.5 V, and green and blue LEDs have anominal V_(f) of about 3.0 to 4.5 V. For a lower voltage application,such as 12VDC or 24VDC MR-16, to achieve the desired light output andmatch the input voltage, it may be necessary to configure the LED chipsin parallel and series. Outside the U.S. and Canada, most countries havea household electricity source of 220V or 230V, thus 80 to 160 LED chipsmay need to be connected in series to match the rectified DC.

By matching the combined forward voltage with the voltage of the inputsource, the power supply for the light engine can be simplified suchthat no bulky, complicated voltage step-up or step-down transformers, orswitching power supply, need to be used in connection with the system; asimple, efficient AC to DC rectified circuitry is sufficient. Thisallows the light engine to be incorporated into compact assemblies—forexample, bulb assemblies that fit into standard light bulb sockets.

LED Devices

Any suitable class of LED device 104 may be used in connection with thepresent invention, including individual die, chip-scale packages,conventional packages, surface mounted devices (SMD), or any other LEDdevice now known or developed in the future. In the embodiment describedin conjunction with FIG. 1, for example, LED devices 104 comprisesurface mount devices having electrical contacts that mount directlyonto the surface of trace pattern 106, e.g., “flip-chip” orsolder-bumped die.

Alternatively, referring now to FIG. 2, the LED devices may comprise LEDchips 204 bonded (via solder bonds, epoxy bonds, or the like) torespective PCB pads 206 wherein each die 204 has two bond-pads forproviding electrical connectivity via wire bond interconnects 202.Optionally, intermediate PCB pads 208 may be used to facilitate wirebonding between individual die. This embodiment shows seven parallelsets of seven die connected in series; however, as described above, theinvention is not so limited, and may include any number of die connectedin series, parallel, or a combination thereof.

FIG. 5 depicts an isometric cut-away view of a single LED device asillustrated in FIG. 2. As shown, substrate 102 comprises a highthermal-conductivity base 504 with an overlying highthermal-conductivity, electrically-insulating material 502. IndividualPCB traces 208 and 206 are disposed on layer 502, and LED die 204 isbonded to PCB trace 206. Wire bonds (not shown) are used to interconnectdie 204 with adjacent die (e.g., using intermediate PCB traces 208).

FIG. 4 shows yet another embodiment of the present invention. Inaccordance with this design, the individual LED die 204 are bonded (viasolder bond or other electrically conductive bond) to a PCB pad 206.Individual wire bonds 202 are then used to connect the PCB pads 206 to abond region on an adjacent die. That is, each LED die 204 includes asingle bond pad, and the backside of the die acts as the secondelectrical contact.

LED devices 104 are manufactured using one or more suitablesemiconductor materials, including, for example, GaAsP, GaP, AlGaAsAlGaInP, GaInN, or the like. The size of selected LED devices 104 may bedetermined using various design parameters. In one embodiment, LEDdevices 104 are 300×300 micron square die with a thickness of about 100microns. Those skilled in the art will appreciate that the invention isnot so limited.

As is known in the art, individual LED devices have particular colorscorresponding to particular wavelengths (or frequencies). One aspect ofthe present invention relates to the ability to use multiple LEDs ofvarious colors to produce the desired color of emitted light. Ingeneral, the set of LED devices mounted on the substrate includes x redLEDs, y green LEDs, and z blue LEDs, wherein the ratio x:y:z is selectedto achieve a white light particular correlated color temperature (CCT).

In general, any number of LED colors may be used in any desirable ratio.A typical incandescent light bulb produces light with a CCT of 2700 K(warm white light), and a fluorescent bulb produces light with a CCT ofabout 5000 K. Thus, more red and yellow LEDs will typically be necessaryto achieve 2700 K light, while more blue LEDs will be necessary for 5000K light. To achieve a high Color Rendering Index (CRI), a light sourcemust emit white light with a spectrum covering nearly the entire rangeof visible light (380 nm to 770 nm wavelengths), such that dark red,light red, amber, light green, dark green, light blue and deep blueshould be placed in the mix.

The present invention allows LED devices with different wavelengths tobe incorporated into the light engine in order to achieve these goals.In one embodiment, for example, the mixing ratio (with respect to numberof LEDs) of R (620 nm):Y (590 nm):G (525 nm):B (465 nm) is 6:2:5:1 toachieve 3200K light. In accordance with another embodiment, a R:Y:G:Bmixing ratio of 7:3:7:2 is used to achieve 3900K light. In yet anotherembodiment, a ratio of 10:3:10:4 is used to achieve 5000K light. Thespectra for each of these three embodiments is shown in FIG. 11.

It will be appreciated that the cited mix ratios are dependant on theintensity of the chips as well as their wavelengths. Accordingly, thepresent invention is not limited in the number of types of LEDs thatcould be used to build a desired light output.

In addition to white light, the present invention may be used to produceparticular colors of light using similar color blending techniques. Thatis, while it is often possible to use a number of single-color LEDs toproduce the desired color, it is also desirable in some instances to usetwo or more colors of LEDs combined to form a composite color.

More specifically, due to the material properties of LED compoundsemiconductors, the efficacy of certain wavelengths is undesirable. Forexample, no traditional compound semiconductor materials can emit yellowlight at 575 nm efficiently. This wavelength, 575 nm, is located at theperformance valley between AlGaInP and GaInN semiconductors. By mixingLED devices fabricated from both of these materials, however, yellowlight with the desirable efficacy can be produced.

Substrate

Substrate 102 comprises any structure capable of providing mechanicalsupport for the LED devices 104 while providing desirable thermalcharacteristics—i.e., by assisting in dissipating all or a portion ofthe heat generated by LED devices 104. In this regard, substrate 102preferably comprises a high-thermal-conductivity substrate.

As used herein, the term “high-thermal-conductivity substrate” means asubstrate whose effective thermal conductivity greater than 1 W/° K-m,preferably greater than about 3 W/° K-m The geometry and material(s) ofsubstrate 102 may therefore vary depending upon the application. In oneembodiment, substrate 102 comprises a metal-clad PCB, for example, theThermagon T-Lam or Bergquist Thermal Clad substrates. These metal cladPCBs may be fabricated using conventional FR-4 PCB processes, and aretherefore relatively cost-effective. Other suitable substrates includevarious hybrid ceramics substrates and porcelain enamel metalsubstrates. Furthermore, by applying white masking on the substrate andsilver-plating the circuitry, the light reflection from the substratecan be enhanced.

Encapsulant Layer

A substantially transparent polymeric encapsulant is preferably disposedon the LED devices then suitably cured to provide a protective layer. Ina preferred embodiment, this encapsulant comprises an optical-gradesilicone. The properties of the encapsulant may be selected to achieveother optical properties, e.g., by filtering the light produced by theLED devices. At the same time, this protective encapsulant layer is softenough to withstand the thermal excursions to which the assembly issubjected without fatiguing the die, wire bonds, and other components.

FIGS. 6, 7A, and 7B show various views of one embodiment of the presentinvention wherein the encapsulant covering the LED devices is suitablyrestrained by a dike structure. More particularly, the light engine 100of FIG. 6 comprises an outer dike 602 which surrounds at least a portionof LED die 204. In the preferred embodiment, dike 602 is a generallyrectangular, square, hexagon, round, octagon, or oval structuresurrounding the entire array of LED die 204. Outer dike 602 is suitablybonded to substrate 102 using an adhesive or other desirable bondingmethod. A circular dike is preferred for optical reasons.

As shown, the encapsulant material is preferably deposited over LED die204 such that it fills the volume defined by outer dike 602. That is,referring to the cross-section shown in FIG. 7B (section A-A),encapsulant material 606 is filled to the top surface of outer dike 602.Furthermore, outer dike 602 is preferably fabricated from asubstantially transparent material, e.g., a transparent plastic (e.g.,polycarbonate) material. This transparency will allow emission of lightaround the edges of the light engine.

In an alternate embodiment, a second, inner dike 604 is positioned nearthe center of the LED die 204. Inner dike 604 functions to restrain theencapsulant, and is preferably a transparent material. The presence ofinner dike 604 allows connections to be made through the center of theboard.

Reflector Ring

In an alternate embodiment, the light engine includes a reflector ringwhich substantially surrounds the LED devices and helps to focus and/ordirect the light produced by the system.

Referring to FIG. 8, an exemplary reflector 802 is suitably bonded tosubstrate 102 of the light engine in such a way that the all of the LEDdie 204 are located at the base of the reflector. In the illustratedembodiment, reflector 802 is generally conical-shaped. It will beappreciated, however, that reflector 802 may be parabolic, angular, orhave any other desirable shape and size. As shown, reflector 802 acts asthe outer dyke by restraining encapsulant.

To the extent that reflector 802 is designed to direct and focus lightproduced by the LED die 204, it is desirable that the texture andmaterial of reflector 802 be highly-reflective. In this regard,reflector 802 preferably has a generally smooth, polished, mirror-likeinner surface.

In applications where a substantially white light (or other particularcolor) is targeted, and where two or more colors of LEDs are used incombination to produce that color, it is preferred that the innersurface of reflector 802 act to diffuse the light produced by the LEDdevices so as to provide optimal color blending, even though theefficiency or focus of the light engine might thereby be slightlyreduced (due to light scattering). Accordingly, in applications wheretwo or more LED colors are used, the inner surface of reflector 802 ispreferably textured through a suitable process and at a suitable scale.For example, reflector 802 may be faceted, sand-blasted, chemicallyroughened, or otherwise textured to provide the desired diffusivity.Furthermore, the texture or facets may be random, regular, stochastic,or a combination thereof.

Additional Optical Components

In accordance with a further embodiment of the present invention, one ormore optical components are provided on the surface of the encapsulantto provide a desired optical effect with respect to the light beingemitted by the LED devices. These optical components, which maythemselves be a hard glass or plastic, do not pose a danger to the LEDdevices as the encapsulant layer acts as a protective surface. Suitableoptical components include, for example, various lenses (concave,convex, planar, “bubble”, fresnel, etc.) and various filters(polarizers, color filters, etc.).

FIGS. 10A, 10B, and 10C show top, cross-sectional, and isometric viewsof a light engine in accordance with one embodiment of the presentinvention wherein the light engine incorporates a “bubble” lens. More abubble lens 102 includes a flat side interfacing with encapsulant 606,and a bubble side comprising multiple convex regions 1004. In theillustrated embodiment, bubble lens 102 includes a 4×4 grid of suchbubbles. The present invention contemplates any number and size of suchlens features.

Conclusion

In brief, the present invention provides a novel, high-efficiencymulti-chip-on-board LED light engine capable of which may be used in anyconceivable lighting application now known or developed in the future.For example, such light engines may be used in applications calling forlight bulbs fitting into standard household fixtures (standard screw-inbulbs, fluorescent bulbs, halogen bulbs, etc.), automotive applications(tail lights, head lights, blinkers, etc.), portable lightingapplications, and traffic control applications (traffic signals, etc.).Furthermore, the claimed light engines may be used in applicationscalling for a particular color or range of colors, including white lightof any desirable color temperature. Nothing in this application isintended to limit the range of application in which the invention may beused.

Other advantages and structural details of the invention will beapparent from the attached figures, which will be well understood bythose skilled in the art. The present invention has been described abovewith to a particular exemplary embodiment. However, many changes,combinations and modifications may be made to the exemplary embodimentswithout departing from the scope of the present invention.

1. A light engine comprising: a high thermal conductivity substrate; aplurality of light-emitting-diode (LED) devices mechanically connectedto said substrate, said LED devices electrically interconnected in acircuit having first and second terminals, said first and secondterminals configured to accept an input voltage; an outer dike fixed tosaid substrate and surrounding at least a portion of said LED devices; asubstantially transparent polymeric encapsulant disposed on saidplurality of LED devices and restrained by said outer dike.
 2. The lightengine of claim 1, wherein said LED devices are electrically configuredin series.
 3. The light engine of claim 1, wherein said LED devices areelectrically configured in parallel.
 4. The light engine of claim 1,wherein said LED devices are partitioned into sets of serial LED devicesand said sets are configured in parallel.
 5. The light engine of claim1, herein said high thermal conductivity substrate comprises ametal-clad printed circuit board.
 6. The light engine of claim 1,wherein said encapsulant comprises optical-grade silicone.
 7. The lightengine of claim 1, wherein said encapsulant is substantially flush witha top surface of said dike, and wherein said dike is substantiallytransparent.
 8. The light engine of claim 1, further comprising an innerdike positioned substantially in the center of said substrate.
 9. Thelight engine of claim 1, wherein said substrate is coupled to areflector ring.
 10. The light engine of claim 9, wherein a portion ofsaid reflector ring serves as said outer dike.
 11. The light engine ofclaim 1, further comprising an optical component provided on saidpolymeric encapsulant.
 12. The light engine of claim 11, wherein saidoptical component comprises a lens.
 13. The light engine of claim 12,wherein said lens is selected from the group consisting of convex,concave, and planar.
 14. The light engine of claim 11, wherein saidoptical component comprises a filter.
 15. The light engine of claim 1,wherein said plurality of LED devices comprises ratios of individuallyselected LED colors selected to achieve a substantially white light witha target correlated color temperature (CCT).
 16. The light engine ofclaim 15, wherein said CCT is between approximately 2000 degrees K and10000 degrees K.
 17. The light engine of claim 1, wherein said pluralityof LED devices comprise ratios of individually selected LED colorsselected to achieve a target color.
 18. The light engine of claim 9,wherein said reflector ring includes a reflective plating and a textureconfigured to mix light.
 19. An LED light engine comprising: a substratecomprising a metal-clad printed circuit board; a plurality oflight-emitting-diode (LED) semiconductor devices mechanically connectedto said substrate, said LED devices electrically interconnected in acircuit having first and second terminals, said first and secondterminals configured to accept an input voltage; a reflector ring fixedto said substrate and surrounding at least a portion of said LEDdevices; a substantially transparent polymeric encapsulant disposed onsaid plurality of LED devices and restrained by said reflector ring. 20.The light engine of claim 19, wherein said plurality of LED devicesinclude a first LED having a first color, and a second LED having asecond color, and said reflector ring has a surface texture selected toscatter light produced by said first and second LEDs to facilitatemixing of light from said first color and said second color.